Inductance network



1932' A. J. CHRISTOPHER 1,874,242

- INDUCTANCE NETWORK Filed May 25. 1929 FIG. 3

J0 n J/ H 52 WSU W TWIN W T T T INVENTOR AJ CHRISTOPHER ATTORNEY Patented Aug. 30, 1932 UNITED sT'ATEs nnmun a. cnnrsrorn'mor zouxrzas, NEwYonK, ,assronon T BELLFrELErHom,

LABORATORIES, mcoaromrnn, YORK V or ,NEW YORK, N. Y., A. conromrron or nnw-.,

mnncmncn NETWORK Application filed May 25,

This invention relates to electrical react ance networks and more particularly to balanced inductance networksf reduce the number of coils required in a threeterminal inductance network. Another object is to reduce the weight and size of a three-element inductance network.

A further objectis-to increase the degree of balance. obtainable in a. three-element inductance network. In effecting theseobjects, use is made of' the mutual induction existing between two: coil windings wound on the same-core. It

has beenshown in the articleMutual inductance in wave filters with an introduction.

in filter design b K. S. Johnson and T. E. Shea in the Bell ystemj Technical ournal, January, 1925, that a two-winding coil or transformer is electrically equivalent to a star or to adelta network 'ofthree inductances. Ifthetwo coil windingsare equal and connected series-aiding,-theequivalent star network is a symmetrical Tstructure,

5 the shunt branch of which is a negative in-;

dnctance and the two series branchesi'equal positive inductances. The terminals of the structure are constituted bythejunctlonsof the windings; If, furthermore, the coupling branch of the equivalent T will have a negative inductance :precisely equal llnmagnitude.

the equivalenttT. will have positivexinduc each winding and will be very accurately balanced. This condition of unityqcoupling iscloselv approximated ineith'er a'bifilar or aninterleaved winding on atoroidal core composed of a material having-ihi h permeability. This, invention contemp ates connecting a single winding, or two-terminal,

coil of known valueto-thejunction Joint" v i pair of equal szed conductors, each one of of the double-winding coil .Whereby it is possible to assign to'the'shunt arm, of the equivalent T network any desired posit ve inductance value, and any value of'negative inductance not greater than the self-inducthe two windings and the two free. ends of in. acc'ordance with the invention between the two" windings'begp'erfect; i:"e.,'; the coefficient of coupling unityythetshunt to the positive selfrinductance of each of' the windings. while the series branches of other inthe series-aiding connection, that is,

1929. Serial 110. 365,965. a

t ance of each winding. From this, it fol- 4 flows that the inductance between the free The principal object of the invention is to terminal of the single-winding coil and either of the freejterminals of the doublewinding coil can be made any desired positive value.

Tht usual method of obtaining a symmetrical T network is by the use of three separate coils having no appreciable mutual inductance-B0 betwe'enlanytwo. In accordance with the present invention it, is possible to arrive at the same result by using only two coils,i. e.,

two separate cores. Furthermore, without making use of mutual lnductance it is not possible to obtain a structure whoseequivalent T, network has a negative inductance branch nor is it possible to obtain as high a degree of impedance balance between the two series in-, ductance'sof the T equivalent. Coils having mutual inductance so disposed that the structure is equivalent to a T-network comprising a negative inductance branch," having reviously been suggested and constructed, at in these cases the magnitude ofthe negative inductanc'e has been controlled by regulating the coefiicient of coupling, whichis not easily accomplished with great accuracy.

Fig.1 shows in s hematic form, a network I Fig.2 is a schematic d agram of a Tnetwork ele'ctrically equivalent to that in Fig. l;

3 is a schematic diagram of a phase er in wliich are in luded networks inac v-rithjih s l"Cl1ti i3]. r

; :ho nvention is i 'stwtcd in F g.1, in wh'ch 8 is adoubie vfindin 7'. inductance oil d 9 's a single-winding coil which is not magnetically coupled withccil 8. C0518 com prises a toroidal magnetic core 10 on which are wound two equal wind ngs .11 andlQ of insulated electricalconductor. The core materialmay be any high permeability substan e such 'as silicon steelor a nickel-iron alloy. The windings consist preferably of a twisted the pair constituting a bifilar winding. .An end of one winding is joinedto the end ofthe Itis well known that a coil such as 8 iselectrically equivalent to a threebranch T, or

star, connected network of inductances in which two of the branches are equal positive inductances and the third branch a negative inductance. Equivalence of networks as here used, meansthat one network can be substitute'd for the other in any: system without affecting the current and voltage relations in any otherpart of the system. A'systemequivalent-to that in-Fig. 1 is shown in Fig. 2 in which inductancesA, A and B constitute the 'T network equivalent to coil'8. The terminals are numbered 1, 2, 3 and 4 to correspond to the same numbered terminals in Fig. 1. The' inductance 9 is shown connected at terminals 3, as in Fig. 1. The shunt arm, consisting of inductances B and 9 in series, is designated C. Since the core 10 has a high permeability,

fthere is practically no magnetic leakage and the coefficient of coupling between the two windings 11 and 12 is practically unity.

Consequently, the mutual inductance, which windings. The inductances of the equivalent T network bearthe relations:

When the two windings 11 and 12 are; double-winding coil having equal windings forming series inductances and oined in the series-aiding connect1on,-the coefficlent of inductive coupling between. them being subwound bifilarly with the same kind of confrequencies. The invention is not limited to a bifilar winding, however. A coil in which the two windings are applied separately, but

have their portions interleaved with each other can be made to furnish a fairly accurate impedance balance and a coefficient of coupling practically equal to that of a bifila-r, wound coil.

Connecting coil 9 at its terminal 3 has the effect of connecting the positive inductance,

which will be called L of coil 9 in series with the negative inductance B of the equivalent a T network. Since L and B are additive, the

the system of Fig. 1 is to be made equivalent,

it is simply necessary to make use of formulae (1) and (2), From formula (1) it follows that the selfinductance L of each of-the the invention has application. Jan electrical'phase shifting network com- .will be called M, is practically equal to the iposltlve' self-inductance, called L, of each of the equal equal windings 11 and 12 should be equal to half of the required inductance A of each of the series arms of the T equivalent. From formula (2), it follows that without the addition of L the shunt arm of the T has a negative inductance equal to L. Consequently, the inductance'L must be of such a value that the quantity (I -B) is equal to the required inductance of the shunt arm. The inductance between the shunt terminal and either one of the series terminals is equal to (+L so if it is desirable toassign a value to this inductance it is simply necessary to choose a value of L accordingly.

Fig. 3 is an example of a system in which This shows prising three bridged-T sections 30, 31 and 32, eachsection including a T,-or star, system of inductances. In section 30, the branches of the inductance network are designated A,

A and Cto-correspond with the branches in Fig. 2. Obviously, the equivalent of this inductance network can be supplied by the systerm in Fig. 1.v

The invention is applicable in other systems such as electrical filters and attenuation equalizersas well as in phase shifters. In

, phase shifters,-such as that of Fig. 8, the shunt inductance C is generally negative,

while-in filters and equalizers, it is usually What-is claimed is:

1. A four-terminal transmission network of the bridged-T type,- said network including a bridging impedance and a T-network, said -T'-'netWork:comprising in series-shunt comstantially unity,-:said single-winding soil having one terminal connected'as a shunt imwpedanceto the junction'of the two windings :of-said double-winding coil.

2. :A wave transmission network having a pa r ofzinputtermmalsand a pair of output terminals, Sfild network comprising electri cal paths-between each inputterminal and a respectively associated output terminal, a

pair of equalv inductances included in series in one of said paths, an impedance network connected in parallel with said inductances between the outer terminals thereof, and a .bifilarly on a toroidal core of relatively high magnetic permeability, and said pair of inductan'ces being joined in theseries-aiding I connection.

3. In a wave transmisison network of the bridged-T form, said network having a pair of input terminals and a pair of output terminals, a plurality of electric paths between an input terminal and a respectivel associated output terminal, one of said pat s comprising a pair of equal inductances, and an impedance network joined to the junction of said pairof equal inductances, said impedance network comprising athird inductance,

said pair of equal inductances being of the interleaved type wound on a toroidal core of relatively high magnetic permeability, and said pair of equal inductances being joined ,in the series-aiding connection.

4. In a bridged-T network comprising a bridging impedance and a T-network, a combination of inductance coils forming said T- network, said combination consisting 7 of a double-winding inductance coil and a singlewinding inductance coil, said double-winding coil comprising equal windings joined in the series-aiding connection and having a COQlfiClGIlt of inductive coupling between them substantially equal to unity, whereby the inductances of thevseries branches of said T- network are equivalent to each other and to ,substantially twice the self-inductance of each winding, said single-winding coil having one end connected to the junction of the two windings of said double-widing coil.

In witness whereof, I hereunto subscribe my name, this 23 day of May, 1929.

ARTHUR J. CHRISTOPHER. 

